Polyolefins such as propylene polymers or propylene-α-olefin copolymers are inexpensive and yet excellent in mechanical properties, heat resistance, chemical resistance, water resistance, etc., and thus, they are used in various fields. However, such polyolefins have no polar groups in their molecules and thus are usually of low polarity and difficult to bond or apply for coating, and their improvements have been desired. Accordingly, various methods have been tried such as a method for chemical treatment of the surface of a polyolefin molded product with a reagent or the like, or a method for oxidation treatment of the surface of a molded product by such a means as corona discharge treatment, plasma treatment or flame treatment. However, such methods not only require special apparatus but also the effects for improving the coating property or the adhesive property have not been necessarily adequate.
Therefore, in an attempt to impart a good coating property or adhesive property to a polyolefin such as a propylene polymer by a relatively simple method, a so-called chlorinated polypropylene or an acid-modified propylene-α-olefin copolymer, and further an acid-modified chlorinated polypropylene have been developed. Such a modified polyolefin is applied to the surface of a polyolefin molded product as a surface treating agent, an adhesive or a coating material. The modified polyolefin is usually applied in the form of a solution in an organic solvent or a dispersion in water. With a view to safety and health, and reduction of environment pollution, an aqueous dispersion is usually preferably employed.
Such an aqueous dispersion may, for example, be a resin dispersion having an acid-modified chlorinated polypropylene dispersed in water by using a surfactant and a basic substance (Patent Document 1) or a resin dispersion having an acid-modified polyolefin dispersed in water by using a surfactant and a basic substance (Patent Document 2). However, such a resin dispersion has had a problem that in order to make dispersed particle sizes to be fine, it is required to add a large amount of a surfactant, and consequently, a coating material employing such a resin dispersion is poor in water resistance or chemical resistance. Further, after coating, the surfactant may sometimes bleed out on the coated surface. On the other hand, if the amount of the surfactant is reduced, the particle sizes of dispersed resin tend to be large, and there has been a problem in storage stability. In an emulsified system using a surfactant, it has been difficult to satisfy all of such requirements, and a further improvement has been desired. Further, in Patent Document 1, a chlorinated polypropylene having the melting point made relatively low is employed to secure the dispersibility of the resin, but with a view to preventing environmental pollution, it is desired to reduce the amount of chlorine to be used.
As another example, there is a resin dispersion having dispersed in water a polymer having functional segments block-copolymerized to polypropylene segments (Patent Document 3). However, the dispersed particle sizes of the ethylene copolymer or polypropylene homopolymer can not be said to be sufficiently fine, and it is not possible to form the dispersion without using a surfactant at all. Thus, a further improvement has been desired.
Under the circumstances, a polyolefin aqueous dispersion has been proposed wherein a polyolefin having a large amount of an unsaturated carboxylic acid grafted to a propylene/α-olefin copolymer, is dispersed in the presence of a base (Patent Document 4). However, since a propylene/α-olefin copolymer having a wide molecular weight distribution and having copolymerizability not controlled, is used, it is required to use a pressure resistant vessel and to use a special method for dispersion at a temperature of at least the boiling point of water or the solvent, in order to make the dispersed particle diameter to be fine.
Further, an aqueous dispersion has been proposed wherein a maleic anhydride-modified chlorinated propylene/ethylene copolymer having a chlorinated propylene/ethylene copolymer modified with maleic anhydride is dispersed in the presence of a base (Patent Document 5). However, the molecular weight of the raw material to be used is very low at a level of 6,500, whereby the aqueous dispersion is inferior in the adhesion to the polyolefin substrate, water resistance and chemical resistance.
On the other hand, as a propylene/α-olefin copolymer to be used, it is proposed to use a propylene/α-olefin copolymer having a narrow molecular weight distribution and a copolymer arrangement being random and uniform, which is prepared by a metallocene catalyst, instead of a propylene/α-olefin copolymer which is prepared by means of a Ziegler-Natta catalyst (Patent Document 5). It is disclosed that it is thereby possible to obtain an aqueous dispersion excellent in the low temperature heat sealing property. However, it is still essential to use a surfactant, the stability is poor with large particle sizes, and there still remain such a problem as bleeding out due to the surfactant or poor chemical resistance.
Further, an aqueous dispersion is also proposed wherein no α-olefin is used, and only a polypropylene having a stereo-block structure is used (Patent Document 6). However, it is still essential to use a surfactant, and there still remains such a problem as bleeding out.    Patent Document 1: JP-A-10-231402    Patent Document 2: JP-A-6-256592 (U.S. Pat. No. 5,534,577)    Patent Document 3: JP-A-2001-288372 (U.S. Patent Application Publication No. 2003-055179)    Patent Document 4: JP-A-2005-126482    Patent Document 5: JP-A-3-182534    Patent Document 6: JP-A-2000-344972    Patent Document 7: JP-A-2004-002842 (U.S. Patent Application Publication No. 2005-124753)